Manufacture of traveling wave tubes



Dec. 4, 1956 J. P. Alco MANUFACTURE OF rRAVELING WAVE TUBES Filed Oct.2l, 1953 ATTQAWEV /Nl/ENTOR J. l? LA/CO 5v i United States Patent OMANUFACTURE OF] TRAVELING WAVE TUBES Joseph P. Laico, Springfield, N.J., assigner to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application October 21, 1953, Serial No.387,362 i 2 Claims. (Cl. S16-18) This invention relates to travelingwave tubes and more particularly to the manufacture of such tubes.

It has been found that traveling wave tubes having .very small helicesfor operationat frequencies of the order of thousands of megacycles,such as in the range of 4,000 to 11,000 megacycles or higher, exhibit afading of output power after the tube had been 4operated for a fewminutes. This power fade, as more fully discussed in my applicationSerial No. 387,361 filed October 21, 1953, is caused by increase in theresistive loss on the helix above that intrinsic to the `helix and theattenuation deposited thereon. This increase results from heating ofhelix due mainly to radio frequency power traveling along the helixtransmission line in excess of the transmission capacity of the helix.This excess power on the transmission line occurs adjacent the outputend as the radio frequency voltage and power on the helix increaseexponentially along the helix.

As disclosed in the above-mentioned application,V a4

proper balance can be found at high frequencies between output power,gain and efhciency, all of which are related to the helix size, byhaving the portion of the glass envelope adjacent the output or electroncollector end of the tube in intimate contact with the helix supportrods over a significant portion of their area. A l

It is desirable, however,y that the glass envelope be collapsed onto thehelix support rods only adjacent the output end and that it not be incontact at any point with the wires of the helix itself extendingbetween support rods, as discussed in the above-mentioned application.

It is a general object of this invention to facilitate the manufactureof traveling wave tubes.

Further it is an object of this invention to provide an improved methodfor collapsing a limited and predetermined portion of an elongated glassenvelope onto the helix support rods of a traveling wave tube.

It is another object of this invention to prevent undue thermal strainin the glass envelope of a traveling wave tube between the collapsed anduncollapsed portions.

It is a still furtherv object of this invention to provide an improvedmethod for outgassing helices of traveling wave tubes. Morespecifically, it is an object of this invention to provide a method ofoutgassing helices that is easilycontrollable, eicient, and reduces thenumber of tubes damaged during outgassing of the helices.

Theseand other objects of this invention are attained in a specificillustrative embodiment in which a helix mounted on helix support rodsis slid into an elongated glass envelope portion to make a close ttherewith. A continuous current pathfis provided through the helix whenpositioned in the traveling wave tubefby a lead electrically connectingthe input end of the helix to one of the thermal pins in lthe base ofthe tube and a fuse wire electrically connecting the output end of thehelix to the, electron collector of the traveling wave tube. To collapsepredetermined portions of the glass envelope onto the helix support rodsin accordance with an aspect of this invention, a current is passedthrough the helix along this direct current path while the tube is beingevacuated. This current will heat the glass but will not sufiicientlysoften the glass to allow the atmospheric pressure to collapse the glassonto the support rods. A reflecting cylinder` is positioned around theglass to give the additional temperature rise needed to soften the glasssuiciently to allow the pressure differential to deform it and collapseit onto the support rods. The heat reflecting cylinder may be longenough to deform the entire length desired or may be moved along thehelix to attain any desired lengthof collapse of the glass envelope.

Either following the collapse of the glass envelope oi'y prior theretobut without the heat reflecting cylinder encompassing the envelope,acurrent is passed through the helix to outgas the helix. By providingthe heat for outgassing the helix by passing a current through thehelix,`

rather than by the prior techniques of inductive heating the helix,`more ellcient and safer outgassing of the helix may be attained asbetter control of the heating of the helix is possible.

When the helix has been outgassed, the envelope is evacuated, a shorthigh current `passed through the helix to burn out the fuse thatprovided the continuous current connection from the helix to theelectron collector and It is a further feature of this invention that acurrent be passed through the helix during the manufacture of t atraveling wave tube to heat the helix and the glass envelopeencompassing the helix, that the envelope be evacuated, and that a heatreflecting cylinderbe positioned around the glass envelope adjacent theportion to be collapsed onto the helix support rods to provide theadditionalheat necessary to soften the glass sufficiently to allowcollapse underthe differential pressure existing on the'glass envelope.

It is another feature of this invention that the helix of a travelingwave tube be outgassed by passing a current through the helix and a pathincluding a fuse wire within the traveling wave tube and that the fusewire be burned out following the outgassing the helix.

A complete understanding ofthis invention and of these and otherfeatures thereof may be gained from consideration of the followingdetailed description and the accompanying drawing in which:

Fig. l is an external view of a traveling wave tube having a portion ofthe elongated vglass envelope collapsed onto the helix support rods;

Fig. 2 is a sectional view of the tube of Fig. l showing the tube in theprocess of manufacture after the glass envelope portion has beencollapsed but before the helix fuse wire has been ruptured;

Fig. `3 is asecti-onal view of the helix and collapsed envelope portiontak-en along the line 3-3 of Fig. 1;

Fig. 4 is azschematic representation of the circuit and equipmentemployed in the collapsing of the glass envelope in accordance with anaspect of this invention; and

Fig. 5 is an enlarged sectional view of the output end of the helixsupport rods and the electron collector showing particularly the fusewire connecting the helix and the electron 'collector during themanufacture of the traveling wave tube in accordance with the processesof this invention.

Turning now to the drawing, Fig. 1 is an external side viewgof aVtravelingwave tube having a portion ofthe elongated glass tubing 10between the electron gun assem-1 Patented Dec. 4, 1,956

bly 11 and the `collector assembly 12 collapsed onto the helix supportrods 14, as seen best in Fig. 3. The electron gun assembly 11 may be ofany known type; however, for reasons set `forth below, a current path isprovided from the helix 15 through the gun assembly to one of theterminal pins 17 in the base of the assembly. The particular gundepicted in'Fig. 2 comprises a plurality of electrodes 18 which aremounted by support pins 22 which extend through apertures 20 in theinsulating ring 21. These electrodes are positioned coaxally by amandrel, not shown, which assures proper alignment of the parts. When somounted a glaze is positioned in the apertures 20 in the insulating ring21 and around the support pins 22 to which the electrodes 18 areconnected and the pins 22 are then glazed to the insulating ring 21.This mandrel is then withdrawn. To assemble the helix to the gun anothermandrel extends through both and the helix connected to "the gun bywelding telescoping tabs on the gun assembly to portions of an aligningsleeve 274 on the helix assembly. The electron collector assembly isconnected to the helix assembly before the latter is connected tothe gunassembly. However, as best seen in Fig. an insulating ring 26 isinterposed between the electron collector cylinder 27 and an innersupport ring 28 bonded to the inner surfaces of the support rods. Thecathode 25 of the gun is then positioned in the gun and helix assembly,appropriate connections made to the bore pins, the bore sealedto one endof Athe tube envelope and the collector assembly to the other end of thetube envelope. In accordance with one aspect of this invention a fuseWise 30 is initially connected between the collector cylinder 27 and thering 28 towhich the helix is electrically connected. There 1s thus, asshown diagrammatically in Fig. 4, a direct current path at this timethrough the tube comprising one of the terminal pins 17, a lead 31connecting this pin to the aligning tube 24, the helix 15, ring 28,fusewire 30, and collector cylinder27. To collapse a portion of theglass envelope onto the helix support rods to enable power operation ofa traveling wave tube at high frequencies, as described in theabove-mentioned application, a current is passed through this path whichis shown in Fig. 4 as being closed by a conductor 32, a switch 33, and abattery or voltage source 34. At the same time the envelope of the tubeis exhausted, as by a vacuum pump 36 connected by rubber tubing 37 tothe collector cylinder 27. The heat generated by the current in thehelix is sufcient to soften the glass of the envelope 10 but not to thepoint where the glass will collapse under the pressure dilerentialexisting between the atmosphere and the evacuated envelope. Inaccordance with an aspect of this invention a heat retiecting cylinder40 having a bright inner surface is placed around the envelope at apoint where it is desired to collapse the glass. The heat reflected backupon the glass by the cylinder 40 is then suicient to give theadditional temperature rise needed to soften the glass to allow theatmospheric pressure to collapse the glass onto the helix support rods14. Y

The total distance of this collapse is very slight, being of the orderof less than a mil as the helix support rods are preferably slid intothe envelope y10 with a close Vtit. It is, therefore, important that theamount of heat and the collapse of the glass be carefully controlled toprevent too great a collapse which would result in the glass being incontact with the helix'itself. As more fully discussed in theabove-mentioned application, if the glass is contiguous to the helix, itprovides a low dielectric path for the electromagnetic energy on thehelix. Energy is vthen shunted through this path instead of passingthrough the center of the helix where interaction of the electromagneticwave energy and the electric beam is attained. When the glass is incontact with the helix, the degree of coupling and the eiciency of thetube are, therefore, reduced.

I have vfound it desirable to control the collapse of the glass envelopeby positioning a thermocouple 42 within the reflector 40 and preferablyin contact with the envelope 10. The leads of the thermocouple may bebrought out through apertures in the reflector 40 and connected to asuitable meter or indicating device 43.

By heating the entire glass envelope 10 due to the current through thehelix 15 While at the same time applying the additional heat needed tocollapse the glass only at limited areas the temperature dilerentialbetween the collapsed and the uncollapsed portions of the glass isminimized with a resultant reduction in thermal stress in the glass.Advantageously the ends of the reflecting cylinder 40 are partiallyclosed further to confine the heat to the desired area of collapse.

The reflecting cylinder 40 may be of sucient length to collapse theentire portion of the glass envelope desired to be collapsed or may besmaller and be moved along the envelope to collapse successive parts ofthe envelope.

After the glass is collapsed along the desired length of the envelope10, the rellecting cylinder 40 is removed and, in accordance withanother aspect of this invention, a current is passed through the directcurrent path identitied above and best seen in Fig. 4. This currentserves to outgas the helix. Priorly helices have been heated byinductive coupling to a high frequency generator. This technique howeverhas not only been awkward and inetticient, but has been exceedinglydifficult t-o control resulting in a large number of rejected tubes. Bypassing a direct current through the helix itself, outgassing of thehelix can be attained with very accurate control on the heating of thehelix and damage to the helix or the tube during this outgassing can,therefore, be prevented.

After the helix has been outgassed and evacuated, the fuse wire 30 isburnt out by application of a short high current to the helix, and thecurrent path through the tube is removed. The lead 31 and terminal pin17 to which it is connected may be utilized during the operation of thetube for applying a direct current voltage to the helix, as is known inthe art, or may be unconnected in the operating circuit. The tubeenvelope is then sealed, as by closing olf the open end of the collectorcylinder 27.

While in the above description `the outgassing occurs after thecollapsing of the portion of the glass envelope, I have found that theorder in which these two steps of the process -occur is not importantand that the helix may advantageously be outgassed rst. In this case theoutgassing current is applied to the helix while the envelope is beingevacuated but before the reflecting cylinder has been placed around theenvelope. After a suicient time for outgassing the reflecting cylinderis positioned around the envelope without interrupting the current orremoving the tube from the pump, and the glass collapsed. Following thisthe current is removed, the tube sealed, and the fuse wire ruptured, asdescribed above.

In one specific illustrative embodiment wherein the helix was rstoutgassed and wherein the helix 15 was of 0.010 inch molybdenum wire, anoutgassing current of 2.89 ampercs is applied to the helix for about 30minutes, raising the temperature of the helix to about 760 C. throughoutthe entire length of the helix. The heat reliector was then positionedlaround the helix and the same current employed for collapsing the glassonto the helix. The heat reflector raises the temperature of the glassenvelope to about 625 C., and the collapse occurs in about 1A minute.The current was then turned oif, the helix cooled to room temperatureand a current of approximately 4 amperes appliedy to the helix torupture or burn out the fuse wire 30, which was a 0.006 inch platinumwire, though other fuse wires known in the art may be employed. The tubeis then sealed.

It is to be understood that the above-described arrange ments are merelyillustrative of the application of the principles of this invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

5 What is claimed is: 1. In the manufacture of traveling wave tubes, the

4 process for obtaining a controlled collapse of an elongated glassenvelope portion onto helix support rods comprising the steps ofslidably inserting helix support rods to which a helix has been attachedinto an elongated glass envelope, evacuating said envelope, passing acurrent through said helix sufficient to heat said helix and saidenvelope but insucient of itself to soften the glass enough to causeatmospheric pressure to collapse the glass onto the helix support rods,reilecting the heat from said helix back upon said envelope adjacent theportions thereof to be collapsed to provide the additional temperaturerise sucien-t to soften the glass for collapse thereof, anddiscontinuing the collapse of said glass after said glass has collapsedonto said helix support rods and before said glass is contiguous to saidhelix.

2. In the manufacture of traveling wave tubes, the process for obtaininga controlled collapse of an elongated glass envelope portion onto helixsupport rods com- 20 prising the steps of slidably inserting helixsupport rods to which a helix has been attached into an elongated glassenvelope, evacuating said envelope, passing a current through said helixand a wire in series therewith and electrically connecting said helix tothe electron collector of the traveling wave tube`- said current beingsufficient to heat said helix and said envelope but insucient of itselfto soften the glass enough to allow atmospheric pressure to collapsesaid glass onto the helix support rods, reflecting the heat from saidhelix back upon said envelope adjacent the portions thereof to becollapsed to provide the additional temperature rise sutcient to softenthe glass for collapse thereof, discontinuing the collapse of said glassafter said glass has collapsed onto said helix support rods and beforesaid glass has collapsed onto said helix, and subsequently passing acurrent through said helix suflicient to rupture said wire.

References Cited in the le of this patent UNITED STATES PATENTS 832,302Ernst Oct. 2, 1906 2,651,100 Grouse Sept. 8, 1953 2,671,873 Meier Mar.9, 1954 FOREIGN PATENTS 476,488 Great Britain Dec. 6, 1937

